Molding methods include those which involve injection molding and extrusion methods where a polymer is heated to a plastic state, then formed in a mold or die, then cooled to form a finished product. As such, the mold is made from a substrate that is able to withstand these pressure and temperature changes. Often, molds are made of high strength steels and are expensive to produce.
For the solution of these problems, UV curing methods have been proposed in the last four decades. According to these methods, ultraviolet radiation is applied to resins such as coatings, adhesives, marking ink and photo-resists, and photocurable substances etc., to cause photo polymerization. UV curing methods can show inter alia the following advantages: i) in the curing reaction, a liquid monomer changes to a solid polymer within a few seconds, ii) since the entire material is basically cured by solvent-free photo polymerization, it can be very effective to fulfil the requirements of environment-related regulations and orders such as PRTR (Pollutant Release and Transfer Register) Law or ISO 14000, iii) the UV curable material does not cure unless exposed to light and, unlike heat-curable material, it does not get cured gradually during preservation, hence, its pot-life is short enough for it to be used in automation processes, iv) a great number of resins are available for use in such UV-curing processes.
However, regarding the manufacture of articles having a more or less complex shape, the following problems can arise.
The resins can be characterized in that they are cured only in regions which are exposed to more than a certain amount of energy radiation. On the other hand, energy radiation typified by UV radiation can be characterized in that it is attenuated while it passes through a resin. As a result, it can be either difficult for energy radiation to penetrate the resin deep enough and at every location, or energy radiation is greatly attenuated or absorbed. Thus, photo-curable resins are cured only in a surface layer having a thickness of several micrometers to several millimeters through which energy radiation can pass, and the deep part and the parts which cannot be reached by the radiation which is emitted in a linear manner from the UV-source remain uncured. Consequently, it can be difficult or partially impossible to apply photo-curable resins to thick-walled materials and/or having a more or less complex shape.
Thus, it is not surprising that UV cure resins have in fact been used to form coatings and adhesives, but they have not generally been used in molding applications. This can, inter alia, be seen from the following examples.
U.S. Pat. No. 5,916,509 entitled “Actinic irradiation and curing of plastic composites within a material forming die” states that uncured thermoset composite reactants containing ultraviolet curable reagents may be effectively cured by pultrusion using a unique molding die, which uniformly irradiates the uncured composites contained within the die.
DE Patent Document No. 197 05 303 entitled “Production of small and micro parts” is concerned with the production of small and micro-parts made of a molding composition and which comprises: (a) evacuating the mold with integrated mold insert; (b) placing the molding composition into the mold under pressure where a low viscosity molding composition is used, which contains a component hardenable by short wave light; (c) irradiating with short wave light while maintaining a post pressure to completely harden the composition; and (d) opening the mold and removing the molding.
In U.S. Pat. No. 5,885,514 entitled “Ambient UV-L-curable elastomer mold apparatus,” a process for molding parts such as gaskets using an injection molding machine is described. According to this document, the molding machine includes upper and lower mold plates, which are transparent to UV light, have pattern recesses of differing sizes, a UV light source, a low pressure injection system for delivering elastomers to the mold plates and are located spaced apart from one another.
In a molding apparatus according to JP Patent Document No. 59215838, an ultraviolet ray hardening molding material is injected into a die from an injection molding machine to fill the space in a cavity core. Then ultraviolet rays irradiate the cavity and the cavity core separately via an optical fiber from an ultraviolet ray irradiation source.